Disc drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disc medium, the actual information being stored in the form of magnetic transitions within the medium. The discs themselves are rotatably mounted on a spindle. The information is accessed by means of read/write heads generally located on a pivoting arm that moves radially over the surface of the disc. The read/write heads or transducers must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of information.
During operation, the discs are rotated at very high speeds within an enclosed housing by means of an electric motor generally located inside a hub that supports the discs. One type of motor in common use is known as an in-hub or in-spindle motor. Such in-spindle motors typically have a spindle mounted by means of two ball or hydrodynamic bearing systems to a motor shaft disposed in the center of the hub. Generally, such motors include a stator comprising a plurality of teeth arranged in a circle. Each of the teeth support a plurality of coils or windings that may be sequentially energized to polarize the stator. A plurality of permanent magnets are disposed in alternating polarity adjacent the stators. As the coils disposed on the stators are sequentially energized in alternating polarity, the magnetic attraction and repulsion of each stator to the adjacent magnets cause the spindle to rotate, thereby rotating the disc and passing the information storage tracks beneath the head.
The use of hydrodynamic bearing assemblies in such drive systems has become preferred due to desirable reductions in drive size and noise generation as compared to conventional ball bearing drive systems. In hydrodynamic bearings, a lubricating fluid, such as oil or air, functions as the bearing surface between a base or housing and a spindle or hub. As the lubricating fluids require small gaps between the stationary and rotating members in order to provide the support, stiffness and lubricity required for proper bearing operation, conventional drive components and assemblies typically require tight tolerances and demand precision assembly methods.
Despite the use of such precise and controlled assembly methods, air bubbles may be introduced into the fluid which supports the relatively rotating services for rotation of the bearing assembly. Thus the problem presented is to establish a reliable bearing design in which the possibility of the existence of air bubbles in the fluid between the relatively rotating grooved hydrodynamic bearing surfaces is diminished.
More specifically, in fluid dynamic bearings, a important goal is low non-repeatable runout (NRR) to optimize tracking and track density. In a fluid dynamic bearing motor, one potential source of NRR is the presence of air in the grooved regions of the bearing, causing lubricant pressure instability and consequential rotor displacement. The presence of air in the bearing lubricant can result from partial fill of the bearing cavity with lubricant or air ingestion due to a combination of conditions including thermal contraction of the lubricant and part tolerances such as cylindrical taper in a journal bearing or symmetrically formed bearing grooves. Due to the lubricant's tendency to flow throughout the bearing due to pressure gradients caused by part tolerances, air bubbles can be swept into the grooved regions of the bearing, resulting in NRR events. Therefore, the problem presented is to adopt a design which eliminates or diminishes the problem of air bubbles being swept into or residing in the grooved bearing regions of a hydrodynamic bearing.